According to the Pirate Bays blog, their database of users has been stolen via, ironically, a security hole in their blog. The database includes usernames, passwords and emails.

"They have got a copy of the user database. That is, your username and passwords. But, the passwords are stored encrypted, so it's not a big deal, but it's still very sad that it's out there. All e-mails are for instance encrypted as well, they will most likely not be able to decrypt them either (they are _very_ encrypted)."

It is suggested that users change their password as soon as possible, especially if their username and password is the same one used on other sites.

Wow, thats probably a lot of users. I imagine if they manage to decrypt the passwords they would be able to get on practically any private site on the net.

Its possible.
However a larger majority of people who use private sites I dont think would be caught dead using TPB anyways. While it has a large selection of torrents the speeds, descriptions, and files themselves ussually suck majorly.
There probably are alot of users of private communities using TPB but not as many as people would probably think.